Development of diazo and azide sensitized colloids



Patented Get. 5, 1954 l'l'ED S ATES PATENT OFFICE DEVELOPMENT OF DIAZO AND AZIDE SENSITIZED COLLOIDS No Drawing.

Application October 4, 1952,

Serial No. 313,191

2 Claims.

The present invention relates to a novel and improved process for the treatment of exposed layers of rotary photogravure tissues which are sensitized with diazo compounds so that printing members of the highest quality may be prepared therefrom.

Objects and advantages of the invention will be set forth in part hereinafter and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the steps, processes and compositions pointed out in the appended claims.

The invention consists in the novel steps, processes, compositions and improvements herein shown and described.

The present invention has for its object the provision of a novel and improved process for the treatment of rotary photogravure tissue sensitized with diazo tanning compound and similar organic sensitizers so that the tissue is not destroyed prior to etching of the cylinder or other printing surface. A further object is the provision of a novel and improved process of treating thin sheets of gelatine or the like which have been differentially hardened in accordance with exposure to light by the tanning action of the tanning compound contained in the sheets and so that the decomposition products of the tanning compound does not impair the quality of the image on the sheet, nor does the sheet suiier any unwanted action during development or etching, such as might otherwise destroy not only the sheet of sensitized and exposed gelatine, but might necessitate the regrinding of the large and expensive copper cylinder onto which the gela- K tine sheet is mounted for the formation of the rotary photogravure printing member.

In accordance with the present invention, the sensitized sheet of rotary photogravure tissue comprises a sheet of thin gelatine, or other gelatinous colloid which may be selectively tanned by means of a photosensitive tanning agent, and which incorporated as the photosensitive tanning agent a tanning compound which is photosensitive, includes the -N:N- group and on exposure to light reacts to produce a compound which renders the gelatine or gelatinous material less soluble in water or less permeable to water. Such compounds, I have fgund, release gaseous nitrogen on exposure to light, and if the gelatinous sheet in which theyware dissolved or dispersed is immersed with water immediately after exposure to light, as is usual in rotary photogravure practice, cause the formation of relatively large blisters in the gelatinous sheet which render the gelatinous sheet unfit for further use and require that another exposed tissue .be prepared, thereby delaying and increasing the cost of the operation of preparing the printing member. Such blister may also appear during the etching process and occassionally cause the gelatinous sheet to become detached from the surface of the printing cylinder in localized areas, thereby destroying the ability of the sheet to control the penetration of the etching solution through the sheet and the localized action of the etching solution of the copper metal of which the printing surface is composed. In this connection, it must be remembered that the perfection of the resulting printing member depends entirely upon the localized action of the mordant on minute areas, each of which is almost always less than 0.01" on each side forming the square area.

The tissues for use in the process of the present invention preferably comprise a sheet of gelatinous material, such as gelatine, from 0.001 to 0.003 in thickness, and usually containing a relatively dark finel divided pigment, which is usually of a brown, dark red or other dark color, and is sensitized with one or more of the photosensitive dyes which can exert a tanning action on the gelatinous material as these dyes are decomposed, so that the exposed areas of the gelatinous material become less soluble or swell less when wet with water during the development process.

Many such dyes exist, and some of them are disclosed in the patent to Zahn, No. 2,100,063 granted November 23, 1937, and in general these tanning compounds include one or the other of the following organic groups:

As will be noticed, either group includes the structure N:N, and either of these groups of the entire light sensitive molecule decomposes on exposure to light to liberate gaseous nitrogen. In the first formula above, it will be noted that there are three nitrogen atoms and these are released on exposure to light, and probably replaced by an OI-I group. In the second formula, where X shows the presence of a monovalent atom such as chlorine, only two nitrogen atoms are released on exposure to light, and the group is likewise probably replaced by an -OH group. It will be understood that only a portion of the formula of the tanning compounds has been shown above, as the remainder of the molecule may vary widely, but the gelatine-tanning compounds include one or the other of the above structures and include the N:N linkage. Such tanning compounds are generally referred to as diazo compounds,

Such a layer of a gelatinous material and a suitable proportion of the tanning compound is exposed to a positive image of the subject to be reproduced, is then exposed to a rotary photogravure screen, after which the gelatinous layer, usually supported on a layer of paper, is immersed in a bath of water to remove the unexposed tanning compound and to swell the exposed and unexposed portions of the gelatine. The wet sheet is then applied to the polished and clean surface of the copper cylinder or other printing member, is tightly pressed thereagainst and the paper layer is removed, after which the mordant is applied. Ferric chloride solutions are exemplary of the mordants usually used and the function of the differentially hardened layer of gelatine is to regulate the diffusion of this mordant towards the surface of the copper cylinder, thereby controlling the etching of the cylinder in accordance with the exposure which has been received by the gelatine sheet. After the etching is complete, the remainder of the gelatine sheet may be stripped from the surface of the printing member by application of an alkaline solution to the gelatine.

Due to the compact nature of the gelatine layer and its relatively strong nature prior to its being wetted with water in the development of the layer, all of the nitrogen gas released during the exposure of the dyed gelatine sheet is retained in the sheet, and as soon as the sheet is wetted during development, the nitrogen causes blisters to appear in the sheet, due to the swollen, weakened condition of the gelatine. These blisters completely destroy the usefulness of the sheet as a diffusion controlling layer for the etching process, and often times several such sheets must be prepared before one is obtained which will produce a satisfactory printing member.

According to the present invention, I have found that these difficulties can be easily overcome and that the formation of blisters can be consistently avoided with the resultant consist- I preferably use a relatively high degree of suction, in excess of 20 of mercury, and while the time of treatment by this degree of suction will vary depending on the thickness of the gelatine layer, the amount of light sensitive compound contained therein, the amount of exposure which the sheet has exposed, the time intervening between the exposure and the wetting or development step, as well as upon the degree of suction used, I find that a treatment of from 5 to 20 minutes is usually sufiicient regardless of the area of the sheet. Thus as a generally preferred step in the treatment of the exposed tanned gelatine layer, I place the sheet in a closed vessel then exhaust the air from the vessel, reducing the pressure within the vessel until a suction of at least 20" of mercury has been established within the vessel. The sheet is allowed to remain with the vessel for the necessary period of time, from 5 to 20 minutes, or more if desired, after which the suction is released, and the sheet is removed from the vessel and may then be developed and further processed in accordance with the usual technique employed in the processing of such material.

The invention in its broader aspects is not limited to the specific steps, processes and compositicns shown and described but departures may be made therefrom within the scope of the accompanying claims without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is:

1. In the process of developing an exposed sheet of colloidal material capable of being tanned and sensitized with a compound containing the N:N group which compound is decomposed on exposure to light to tan the colloidal material and to release gaseous nitrogen, said developing being done by washing with water, the improvement comprising the step of subjecting the exposed sheet to suction prior to the washing of the sheet with water, said suction being of such degree and duration as to remove the gaseous nitrogen from the colloidal material of the sheet.

2. A process as claimed in claim 1 in which the suction is at least equivalent to 20 inches of mercury.

Number Name Date Zahn Nov. 23, 1937 

1. IN THE PROCESS OF DEVELOPING AN EXPOSED SHEET OF COLLOIDAL MATERIAL CAPABLE OF BEING TANNED AND SENSITIZED WITH A COMPOUND CONTAINING THE -N:N- GROUP WHICH COMPOUND IS DECOMPOSED ON EXPOSURE TO LIGHT TO TAN THE COLLOIDAL MATERIAL AND TO RELEASE GASEOUS NITROGEN, SAID DEVELOPING BEING DONE BY WASHING WITH WATER, THE IMPROVEMENT COMPRISING THE STEP OF SUBJECTING THE EXPOSED SHEET TO SUCTION PRIOR TO THE WASHING OF THE SHEET WITH WATER, SAID SUCTION BEING OF SUCH DEGREE AND DURATION AS TO REMOVE THE GASEOUS NITROGEN FROM THE COLLOIDAL MATERIAL OF THE SHEET. 